Dissolved air flotation is one of several processes used to clean and purify water. The process may be used for removing suspended solids, including organisms, from fresh water for municipal water supplies, or for removing suspended solids from industrial wastewater. In the dissolved air flotation process, influent water is mixed with a coagulant to form flocculent particles, and then white water (air-saturated high-pressure water) is injected into a tank of the influent water/flocculent mixture to form air bubbles that attach to the flocculent particles and lift them to the surface. The particles lifted to the surface accumulate in a layer of scum or sludge, referred to as the float. This layer is skimmed off the surface of the tank, while clean water is drawn out of the bottom of the tank.
For municipal water supplies, the suspended solids of concern include microbial pathogens such as cryptosporidium and giardia. Cryptosporidum and giardia are water-borne protozoan parasites that cause intestinal infections. In typical municipal purification systems, chlorine is used to kill these organisms, even after filtration and other purification methods may have removed a large portion of the influent population. This results in the formation of trihalomethanes (chloroform, dichlorobromomethane, dibromochloromethane, and bromoform) when the chlorine reacts with compounds already in the fresh water. These carcinogenic disinfection by-products are clearly undesirable. Reduction of microbial pathogens prior to chemical treatment in the water is desirable in order to minimize the creation of disinfection by-products.
Though dissolved air flotation is promising as a technique for removing microbes and other suspended solids, its use has been hampered by low volume, lack of need given lax water quality standards, and high cost of installation. These limitations are overcome by the new system describe below.